A diverse range of communication systems are in use today enabling communication between two or more entities, such as user equipment and/or other nodes associated with the system. Such systems may comprise, for example, communication of voice, data, and multimedia data.
Communication systems proving wireless communication for user terminals or other nodes are known. An example of a wireless system is a public land mobile network (PLMN). A PLMN is typically a cellular network wherein a base transceiver station (BTS) or similar access entity serves user equipment (UE) such as mobile stations (MS) via a wireless interface. The operation of the apparatus required for the communication is usually controlled by one or more control entities, which themselves may be interconnected. One or more gateway nodes provide for connecting the PLMN to other networks. Examples of other such networks are another cellular network, a public switched telephone network (PSTN) and packet switched data networks such as an IP (Internet Protocol) based network. The communication between the user equipment and the other elements of the communication system are based on an appropriate communications protocol, which defines the “rules” under which communication is handled in the system.
In the current third generation (3G) wireless system, there are defined various servers for the handling of different communication services for mobile users. These include servers that provide call state control functions, known as CSCFs. Control functions may also be provided by entities such as a home subscriber server (HSS) and various application servers. The HSS is typically for permanently storing the user's (subscriber's) profile. For example, in the Release 5 architecture for 3G, as specified by the 3rd Generation Partnership Project (3GPP), these entities can be found located in the IP Multimedia Subsystem (IMS).
The IMS network may sit at the hub of the 3G architecture, supporting an IP based network that handles both traditional voice telephony and multimedia services. The 3GPP has chosen Session Initiation Protocol (SIP) as a core session signalling protocol for 3G networks. SIP has been developed by the Internet Engineering Task Force (IETF). The 3GPP specification 24.229 describing the IMS network basic operation from an SIP perspective can be found at http://www.3gpp.org/ftp/Specs/Latest-drafts/24229-201.zip. It should be noted that SIP is a request/response style protocol, in the sense that for every message sent from a source, there is an associated response from the destination confirming receipt of the sent message.
For example, in a 3G network, when a user first switches on his mobile terminal, he must register his user ID or address with the network before allowing the terminal to fully connect. This is done by sending an SIP ‘REGISTER’ message, which includes details of the users address, from the terminal to the IMS. The IMS processes this information, via the serving call state control function (S-CSCF), storing the relevant registration information at the HSS. This registration information may include the status of the user such as the location, terminal capability and user availability. The registration is acknowledged by the IMS through a suitable response message that is also in accordance with SIP. Subsequent registrations also take place (‘re-REGISTER’) whenever the preceding registration has expired, or when there is a change in the status of the user or another reason to refresh the registration. When a user wishes to set up a session with another user, such as a voice call or sending of a text message, the session negotiation will also be performed under SIP. Typically such a negotiation will use the SIP INVITE message, which is sent from one user to another via the IMS.
Other services, such as instant messaging, local traffic reports, and conferencing facilities, are supplied by application servers (AS) via the IMS. An AS may reside within the IMS network, or outside of it. Typically the AS is external when the service supported is provided by a third party. For example, an AS providing local traffic reports may need the latest information on the status of any users subscribing to that service. As we have noted above, status information can be updated using an SIP re-REGISTER message. The AS server requiring this status information therefore subscribes, using an SIP SUBSCRIBE message, to all the REGISTER messages sent by each and every user subscribing to the instant messaging service offered by the AS. One SUBSCRIBE message is required per subscriber that the AS wishes to receive updates on. The IMS logs these SUBSCRIBE messages, and sends out a NOTIFY message to the AS every time a relevant REGISTER message is received. The AS can then use this information to implement its traffic reporting service.
The reception of a REGISTER message can be classified as an event. Events can be any change of state and associates with an entity, such as a user or another node, in the communication system. Thus an AS can subscribe specifically to REGISTERs, as in the above example, other SIP messages such as INVITEs, or other status changes not specifically associated with an individual SIP message.
Currently, when an external AS or other element subscribes to events in the IMS, it needs to provide the address of a specific IMS element, such as an S-CSCF, that it wishes to send the SUBSCRIBE message to. The internal structure of the IMS is therefore not entirely transparent, and requires the AS to have certain knowledge of where the SUBSCRIBE messages should be addressed. Specifically, the AS requires an exact address for locating the S-CSCF. The IMS operator would normally be reluctant to disclose specific addressing information to the operator of the AS, which would reveal much about the internal configurations of the IMS operator's proprietary system.
An AS may have many users subscribing to the service it provides. In such a situation, the AS would have to send as many SUBSCRIBE messages as it has subscribing users, and also need to know where to send each SUBSCRIBE message as each user may register its status with different elements (S-CSCFs) in the IMS.
It shall be appreciated that although the above discussed problems relate to subscriptions to SIP events in IP based third generation (3G) communication systems, similar disadvantages may associate with other systems as well and thus the description is not limited to these examples.